selftests/bpf: Add selftests for load-acquire/store-release when register number is invalid

syzbot reported out-of-bounds read in check_atomic_load/store() when the
register number is invalid in this context:
    https://syzkaller.appspot.com/bug?extid=a5964227adc0f904549c

To avoid the issue from now on, let's add tests where the register number
is invalid for load-acquire/store-release.

After discussion with Eduard, I decided to use R15 as invalid register
because the actual slab-out-of-bounds read issue occurs when the register
number is R12 or larger.

Signed-off-by: Kohei Enju <enjuk@amazon.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250322045340.18010-6-enjuk@amazon.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Kohei Enju

tools/testing/selftests/bpf/progs/verifier_load_acquire.c

@@ -189,6 +189,20 @@ __naked void load_acquire_from_sock_pointer(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("load-acquire with invalid register R15")
+__failure __failure_unpriv __msg("R15 is invalid")
+__naked void load_acquire_with_invalid_reg(void)
+{
+	asm volatile (
+	".8byte %[load_acquire_insn];" // r0 = load_acquire((u64 *)(r15 + 0));
+	"exit;"
+	:
+	: __imm_insn(load_acquire_insn,
+		     BPF_ATOMIC_OP(BPF_DW, BPF_LOAD_ACQ, BPF_REG_0, 15 /* invalid reg */, 0))
+	: __clobber_all);
+}
+
 #else /* CAN_USE_LOAD_ACQ_STORE_REL */
 
 SEC("socket")

tools/testing/selftests/bpf/progs/verifier_store_release.c

@@ -257,6 +257,20 @@ __naked void store_release_leak_pointer_to_map(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("store-release with invalid register R15")
+__failure __failure_unpriv __msg("R15 is invalid")
+__naked void store_release_with_invalid_reg(void)
+{
+	asm volatile (
+	".8byte %[store_release_insn];" // store_release((u64 *)(r15 + 0), r1);
+	"exit;"
+	:
+	: __imm_insn(store_release_insn,
+		     BPF_ATOMIC_OP(BPF_DW, BPF_STORE_REL, 15 /* invalid reg */, BPF_REG_1, 0))
+	: __clobber_all);
+}
+
 #else
 
 SEC("socket")